In an X-ray machine, the X-ray tube is often a bipolar tube having an anode and a cathode maintained at a high voltage potential. The tube has a filament, a low resistance element, maintained close to the cathode potential and having a controllable current on the order of a few Ampere passing through it to sustain thermionic emission to drive the commanded X-ray tube current. This current is supplied by a filament transformer and the low resistance nature of the filament causes the required high current with application of a voltage on the order of Volts. Therefore, often a 1:1 or a step down design of transformer is used for this application, having primary number of turns equal or higher than the secondary number of turns. The secondary winding is biased to the cathode potential to have the voltage generated by the filament transformer to operate at high voltage potential, a design constrain resulting from the X-ray tube.
Two parameters for measuring the performance of any insulation including transformer insulation are creepage distance and clearance. These are measured between any two metallic points (for example, winding and the core) in the transformer and in relation to the surrounding (for example, chassis) in which the transformer is used.
A high voltage transformer for its intended functioning requires adequate electrical isolation between its windings and with respect to its core. These isolation requirements are achieved by the use of insulated wire on the insulating bobbins that hold the windings on the core. Additional insulation may be provided by the use of insulating papers and polymer tapes between winding layers.
A high voltage transformer is more typically made with a primary winding and a secondary winding concentric with the primary winding, with the primary winding placed close to the core and the secondary winding placed over the primary winding. This arrangement allows adequate insulation between the primary winding and the secondary winding with the use of an appropriate thickness of bobbin. The requirement of a grounded shield for safety purposes is handled with placement of the shield between primary and secondary winding, often directly on the primary winding.
The concentric design of the typical transformer windings discussed above requires substantial window space in the selection of the core. This need for substantial window space is because the window space has to accommodate the primary winding along with its bobbin section and then the secondary winding along with its bobbin section and further to accommodate enough insulation material between the windings and between the secondary winding and the core (the other arm of the core).
For this reason, when it comes to the use of high-voltage transformers specially adapted for use in low power X-ray machines, which typically require light weight and compact packaging, the concentric design of the filament transformer becomes one of the bulky and voluminous option for the X-ray machine. Thus there exists a need to design a filament transformer suitable for bipolar X-ray tubes.